With technological advancement and demand for mobile instruments, demand for secondary cells as an energy source is rapidly increasing. Among such secondary cells, a lithium secondary cell having high energy density and working potential, a long life cycle, and reduced self-discharge is widely used in the related art.
For a lithium secondary cell, lithium ions move from lithium metal oxide used as a cathode to graphite used as an anode at an initial charge stage, in turn, being interposed between interlayers of the graphite anode. In this regard, lithium is highly active and an electrolyte reacts with a lithium salt on a surface of the graphite anode having lithium interposed between the interlayers thereof, so as to generate a particular compound such as Li2CO3, Li2O, LiOH, etc. This compound forms, in general, a passivation layer on the surface of the graphite anode and such a passivation layer is referred to as a solid electrolyte interface “SEI” layer.
The formed SEI layer serves as an ion tunnel to selectively pass lithium ions therethrough which, in turn, prevents side reaction of the lithium ions with the graphite anode or other materials. A charge quantity consumed for forming the SEI layer is substantially an irreversible charge capacity which is not reversibly used for a discharge process. Therefore, the electrolyte is not further decomposed and an amount of lithium ions in the electrolyte is reversibly retained, thereby attaining stable charge-discharge performance (J. Powder Sources (1994) 51:79-104). As a result, when the SEI layer is formed, the amount of lithium ions may be reversibly retained and a lithium cell may have enhanced lifespan properties.
The SEI layer described above is relatively rigid under general conditions for maintaining stability of an electrolyte, that is, at a temperature of −20 to 60° C. and a voltage of not more than 4V, thus sufficiently preventing side reaction between an anode and the electrolyte. However, if a cell is stored in a full-charge state at a high temperature (for example, when the cell is left at 90° C. for 4 hours after completely charging the same at 4.2V), a significant problem such as slow deterioration in durability of SEI film is entailed.
More particularly, when a lithium cell is stored in a full-charge state, an SEI film gradually collapses over time and exposes an anode of the cell, and a surface of the exposed anode reacts with an electrolyte to occur a side reaction and generate gases such as CO, CO2, CH4, C3H6, etc., thus causing increase in internal pressure of the cell.
The SEI layer shows varied characteristics depending on types of a solvent contained in the electrolyte and/or chemical properties of additives and such characteristics are known as a major parameter that influences transfer of ions and charge, in turn, varying cell performance (see Shoichiro Mori, “Chemical properties of various organic electrolytes for lithium rechargeable batteries,” J. Power Source (1997) Vol. 68).
Accordingly, there is a requirement for development of novel techniques to prevent variation in cell performance by adding a desired amount of additive to a cell in order to inhibit adverse effects on transfer of ions and charge while embodying inherent efficiency of the additive.
A major cathode active material for the lithium secondary cell comprises lithium-containing cobalt oxide (LiCoO2). Other substances including, for example, lithium-containing manganese oxide such as LiMnO2 with a layered crystalline structure or LiMn2O4 with a spinel crystalline structure, lithium-containing nickel oxide LiNiO2, and the like may also be considered.
Especially, although LiCoO2 with favorable features such as excellent cycle characteristics is widely used, the above material has some disadvantages such as high price due to inclusion of cobalt as a limited mineral resource and, in turn, restriction in mass application as a power source for electric vehicles, and so forth.
Lithium manganese oxides such as LiMnO2, LiMn2O4, etc. have economic merits of using manganese as a raw material, which is an eco-friendly and abundant natural resource, thus drawing considerable attention as a new cathode active material for replacement of LiCoO2. However, they have some drawbacks such as low capacity, poor cycle characteristics, and the like.
In addition, lithium nickel based oxide such as LiNiO2 is relatively cheap and shows high discharge capacity when being charged to 4.3V and, in particular, a reversible capacity of doped LiNiO2 is near 200 mAh/g exceeding a capacity of LiCoO2 (about 165 mAh/g). Therefore, in spite of its low average discharge voltage and volumetric density, a commercially available battery utilizing an LiNiO2 cathode active material exhibits enhanced energy density. Studies into such nickel based cathode active materials are recently increasing number in order to develop improved high capacity cells.
However, the foregoing LiNiO2 based cathode active material encounters some problems including, for example, a rapid phase transformation in crystalline structure caused by variation in volume during a charge-discharge cycle, drastically reduced chemical resistance when exposed to air and moisture, generation of a large quantity of gas during storage or charge-discharge cycle, etc., thus restricting practical utilization thereof.
In order to solve such problems, a lithium transition oxide wherein nickel in the oxide was partially substituted with any other transition metal such as manganese, cobalt, and the like has been proposed. Such metal substituted nickel-lithium transition oxide has excellent cycle characteristics and capacity properties, however, if a cell is used for a long time, the cycle characteristics are drastically deteriorated and some problems of the cell such as cell swelling due to gas generation, impurities generated due to reduced chemical stability, etc. are not sufficiently overcome. Especially, a lithium nickel transition metal oxide with high Ni content entails problems such as significant cell swelling and poor stability at a high temperature.
Accordingly, there is still a strong requirement for development of improved techniques to solve high temperature stability problems caused by impurities while utilizing a lithium nickel based cathode active material suitable for high capacity cells.
In this regard, as electrolyte additives useful for enhancing cycle characteristics and high temperature stability of a cell, vinylene carbonate, vinylethylene carbonate, and the like have been disclosed. These materials are known to react with an electrolyte to form an SEI film over a surface of an anode.
However, such formed SEI film has relatively low thermal stability and, when being used at room temperature or more for a long period of time or being stored at a high temperature, is decomposed to expose a surface of the anode, in turn deteriorating performance of the cell. Further, if the foregoing material is used in a cell containing nickel-lithium transition metal oxide as a cathode active material, cell swelling and high temperature stability are more seriously deteriorated. Therefore, it is substantially impossible to apply the foregoing additive to the cell.
In this regard, Korean Laid-Open Patent Application No. 2004-0065152 describes a cell having LiCoO2 as a cathode active material, wherein a non-aqueous electrolyte of the cell includes a polyether modified silicon oil with a structure having a polyether chain bonded to a terminal of a linear polysiloxane chain, and ethylene carbonate (EC) mixed with the silicon oil.
Japanese Laid-Open Patent Application No. 2006-086101 describes a cell having LiCoO2 as a cathode active material wherein a polymer electrolyte contained in the cell comprises: (1) a polymer having a main chain consisting of polyolefin, polysiloxane or polyphosphazene and a branched chain with an oxide structure; (2) an additive based on a low molecular weight compound with a molecular weight of 103 to 108; (3) a lithium salt compound; and (4) a cyclic carbonate having an un-saturated group.
According to disclosures by the present inventors, however, even when the foregoing technologies are applied, a cell containing nickel-lithium transition metal oxide as a cathode active material does not have enhanced cycle characteristics and, in addition, high temperature stability and/or cell swelling is substantially not improved.
Accordingly, for application of lithium transition metal oxide with high Ni content as a cathode active material, a novel technique to prevent deterioration of cycle characteristics, capacity properties and/or high temperature stability caused by impurities is required.